(1) Field of the Invention
The present invention relates to thin-film transistors (TFTs), display devices which use the TFTs, and manufacturing methods of the TFTs.
(2) Description of the Related Art
Among actively developed flat panel displays (FDPs), one of common types of display apparatuses is a display apparatus employing organic electro luminescence (EL) elements or liquid crystal display (LCD) elements.
Recently, an organic EL display apparatus using current-driven organic EL elements has caught the attention as a display apparatus of the next generation. In particular, an active-matrix-driven organic EL apparatus uses field-effect transistors (FETs). One of common types of the FETs is a thin-film transistor (TFT). In a TFT, a semiconductor layer is provided on a substrate having an insulating surface. Such a layer works as a channel forming region (hereinafter referred to as a channel layer).
In order to form a TFT to be used for an active-matrix-driven organic EL apparatus, at least the following transistors are required: a switching transistor for controlling driving timing such as on and off of the organic EL element, and a driving transistor for controlling a luminescence amount of the organic EL element. Preferably, the TFT has excellent transistor characteristics, and various studies are on going to achieve such characteristics.
For example, the switching transistor has to further reduce the off-current and decrease the variation between the on-current and the off-current. The driving transistor has to further increase the on-current, and reduce the variation in the on-current.
Furthermore, for example, an amorphous silicon film is used as the channel layer of the TFT. Such an amorphous silicon film, however, has a low mobility, and thus the on-current is low. Thus, in order to ensure the driving performance of the TFT; namely the on-current, studies and developments are proceeding in recent years to achieve crystallization of the amorphous silicon film by heating with a laser beam.
In the case where the crystallized silicon film is used for the TFT, however, the channel layer has left damaged when an ohmic contact layer is formed on the channel layer and processed. This damage leads to deterioration in characteristics of the TFT.
As a technique to reduce the damage to the channel layer when an ohmic contact layer is processed, proposed is a technique to form an insulation film in the TFT as an etching stopper film (See Japanese Unexamined Patent Application Publication No. 2007-305701 for example). Japanese Unexamined Patent Application Publication No. 2007-305701 discloses a TFT having an insulation film working as a protective film, and formed on a crystallized silicon film acting as a channel layer.